This invention relates to a system for shutting down a hydraulic machine, such as a pump, turbine, or pump-turbine, when the gap or gaps between the runner of the machine and stationary part or parts surrounding the runner have narrowed unusually due to deformation, off-center displacement, or any other irregularity on the part of the runner or the like.
The lessening of the gaps occurs, for example, because of the following reason.
With a reversible pump-turbine, switching from the generating to pumping operation requires a high starting torque at no load. It is therefore customary to introduce compressed air into the runner chamber to lower the water level below the bottom of the runner so that the runner can run idle and, after a predetermined runner speed has been reached, allow the water to come up to the level high enough for pumping. If water accidentally enters the runner chamber while the runner is idling, it will find no way out and cause no-discharge operation. When this happens, a water curtain is formed along the stationary parts around the runner, and the runner must run with its blade tips forcing away the water curtain from their path, or with the so-called water-curtain brake applied. This will rapdily lead to temperature increases of the runner and runner chamber and thermal expansion of the runner, with unusual reduction of the gaps between the runner and the surrounding stationary parts.
In an effort to prevent the danger of the runners of hydraulic machines being broken through contact with stationary parts due to deformation or displacement of the runners, a shutdown system using mercury switches has already been proposed. Accordng to Japanese Utility Model Publication No. 31705/61, the proposed system arranges a mercury switch or switches, each comprising two electrodes and a large globule of mercury bridging the electrodes for electric continuity and sealed altogether in a glass container, between the runner and the surrounding stationary parts, in such a manner that, if the runner is deformed of displaced to alter the gap between itself and a given stationary part, it will come into contact with the associated mercury switch and destroy the same. The system detects the malfunction of the runner from the cutoff of the electric circuit including the broken mercury switch, and brings the hydraulic machine to a stop.
With the system above described, the replacement of the broken mercury switch following the operation of the system is not easy, because the switch is usually installed at a point normally submerged. Moreover, such switches using poisonous mercury can pose a pollution problem. A further disadvantage is the extreme difficulty in adjusting the gaps between the runner and the stationary parts since the adjustment work tends to damage the system or mercury switches themselves.